activate, I completely agree with you that teachers should be competent enough to teach bright children right up to Y6. That means that every teacher should be able to comfortably handle L6-7 in all core subjects. That's so not the case now.
Simply being able to do L7 maths is insufficient to be able to teach L7 effectively.
I don't know about sensational TV programs exposing how poor teachers are at maths, but from much experience with teachers (in our family and friends circle and elsewhere) I'd be very surprised if even 50% of them - that's roughly about half 
- can make L7 maths lessons interesting and stimulating.
"I do think a C in too low. But still - Grade C, roughly equivalent to level 5, so should enable most 9 year olds to be taught reasonably well."
Someone with a top achievement of a C in GCSE maths and no further maths training should stick to the reception class and not be allowed out of there on pain of death.
What nobody seems to have challenged is the low level of expectation we have. 4b is considered average for KS2. That's pathetic and dumbed down; the whole system is wrong. The average child is capable of much, much more. My DS is quite good at maths, was L5 in year two and we thought he was a bit of an exception, but now that his sisters are in school I'm realising more and more that there's nothing unusual about him. DD1 and DD2 are very average in their natural ability. They may not be L5 in Y2 but they'll certainly be there by Y4. DS is in Y4 and already has specialist maths teachers from the local secondary because he was just so far ahead, but I dread to think that someone who couldn't get more than a C in GCSE maths a decade or two ago is going to teach DD1 and DD2 when they are in Y6.
L6-7 is the expectation better schools in Asia have of their average pupils at age 11 ...and their pupils achieve it. Bear in mind 40-50 pupils to the class is not uncommon there.
I do believe that the industry needs shaking up. We should be much more selective about whom we as a nation allow to teach our children. And we should be paying a lot more to attract that talent even if that means making sacrifices elsewhere. Dedication to teaching and a love for working with children isn't enough qualification.